There has been explosive growth in the area of wireless communications. A few years ago, the sight of a person speaking into a cellular phone was a curiosity while today, it is commonplace. Communication via cellular phones is supported by wireless telecommunications systems. Such systems service a particular geographic area that is partitioned into a number of spatially-distinct areas called "cells." Each cell usually has an irregular shape (though idealized as a hexagon) that depends on terrain topography. Typically, each cell contains a base station, which includes, among other equipment, receive and transmit antennas that the base station uses to communicate with the wireless terminals (e.g., cellular phones) in that cell. Each antenna is characterized by its individual radiation pattern, which determines the signal coverage area and therefore range and shape of the cell.
Due to instantaneous geographic variations in communications traffic, it is desirable, at times, to adjust the geographic coverage of a particular base station. This can be accomplished by dynamically adjusting the antenna radiation pattern. The advantages of such dynamic adjustment, however, have to be weighed against the corresponding implementation costs. To be competitive, this technology therefore has to be cheap, small, and reliable.
Flat-panel array antennas are typically used for base station antennas. The flat-panel array antenna consists of several radiating antenna elements. The radiation patterns are determined by the collective action of all the radiating elements in the array. Usually, the radiation pattern is characterized by a main lobe and side lobes. In most cases, it is desirable to have a very narrow main lobe, also called an "antenna beam", in one or both angular dimensions. The advantage of this is that the antenna beam is very directive, and the angular power density in the main lobe is very high. The enhancement of main-lobe power density with shrinking beam width is also called "antenna gain". Thereby, the number of array elements in each physical dimension and their spacing determines the maximal achievable gain.
In order to obtain a wide variation of radiation patterns for a given antenna array, signal amplitude and signal phase of each individual array element have to be tunable. In real applications, however, only a few basic beam-pattern alterations are important. This reduces the amount of controllable parameters significantly. In most cases, it is sufficient to steer the angular position of the main-lobe ("beam steering"). In a number of applications, it is also desirable to control the beam width of the main lobe ("beamwidth variation").
The beam of an antenna array can be steered by only tuning the signal phase of all radiating elements. If the radiating elements are equidistant, the angular position of the main-lobe is shifted by successively increasing or decreasing the signal phase of one radiating element to the next. If all elements have equal signal phase the beam position is perpendicular to the antenna panel. This is called the "bore-sight" beam. To steer the beam by an angle .alpha. from its bore-sight position, the successive phase increase from element to element .DELTA..phi. is given by: EQU .DELTA..phi.=2.pi..multidot.(l/.lambda.).multidot.sin(.alpha.)(1)
Here, l is the element spacing and .lambda. the free-space wavelength of the transmitted or received signal.
A beam-width variation is obtained by dividing the array into two halves ("sub-arrays") and to steer the beam of each sub-array in an opposite direction. The signal phase thus successively increases, or decreases, from the middle of the total array to both ends. This procedure widens up the beam, if applied in a sufficient amount. It also leads to ripples in the main lobe. In most applications, however, these ripples are of no concern and this procedure is therefore satisfactory. Both procedures, beam steering and beam-width variation, can easily be overlaid.
The implementation of beam-steering and beam-width variation into an antenna array depends on the particular type of feed network used. There are two principally different types of feed networks: the corporate feed network and the series feed network.
For a corporate feed network, the aforementioned beam-shaping capabilities require a separate phase-shifter in each branch that leads to a radiating element. Since beam steering requires a successive increase of phase-shift from element to element, the tuning range per phase-shifter grows with the amount of array elements. For an n-element array, a maximum tuning range of (n-1).multidot..DELTA..phi., or at least 360 deg, is required for the last element. For most applications, this is impracticably large.
For a series feed network, the phase-shifters can be implemented into the main branch of the network. The signal going to the n.sup.th element, therefore, passes (n-1) phase-shifters. This has the advantage that each phase-shifter has to have a tuning range of .DELTA..phi. only. Therefore, all phase-shifters can have the same design.
In such a series feed network, the phase-shifters are connected to the signal side branches via additional transmission-line sections with a corresponding electrical length .beta.. This additional phase .beta. also adds up successively from element to element. In most cases, the feed network is laid out such that .beta. becomes multiples of 2.pi., and .beta. is therefore of no relevance. If .beta. is different from multiples of 2.pi., fine adjustment can be accomplished in the side branches that lead to the antenna elements.
One problem of series feed networks is that the beam position is frequency dependent. Reason for this is that the inter-element signal phase, .beta.+.DELTA..phi., clue to phase-shifter (.DELTA..phi.) and bare signal line (.beta.), grows proportional with the signal frequency. Therefore, changing the signal frequency has the same effect as steering the beam by altering .DELTA..phi.. This limits the bandwidth of a series feed, given by the maximum tolerable variation of the beam position from its target value. In a 5-element array with a spacing of 0.7.lambda., for instance, a frequency change of 6% leads to a beam tilt of 5 degrees.
This problem can be eliminated, when a series feed network is fed in the center of the array. Beam steering requires phase increase per phase-shifter in one half array and phase decrease in the other half array. A frequency variation df now leads to phase increase or decrease of .beta.+.DELTA..phi. in both sub-arrays, i.e. a beam steering of both sub-arrays in opposite direction. This does not affect the beam position since both tilting effects cancel each other out. Therefore, the frequency response of the array is much better.
Phase-tunable series networks seem to offer the appropriate solution for implementation of beam-steering and beam-width alteration capabilities into an antenna array. However, the realization has inherent drawbacks that can make this solution completely unattractive. Specifically, the limited performance of particular network circuits are highly enhanced due to their periodic reoccurrence in the array and when they are spaced such that a resonant condition exists. In a fixed series network, this resonant condition can be avoided by choosing the right phase between the repeated circuits in question. In an adjustable series network, the inter-element phase-tuning requirement makes this resonant condition inevitable since the inter-element phase is subject to changes over a wide range.
Furthermore, the most problematic network circuit is the phase-shifter itself, since it is difficult to match it sufficiently over a wide tuning range. FIGS. 1a and 1d, respectively, show an example of a 5-element tunable series feed and its performance degradation due to the implemented phase-shifters. The phase-shifter return-loss has been set to -21 dB (2 GHz), which is considered a good match (an equivalent circuit is presented in FIG. 1b). The return loss of the array, however, is 10 dB worse for particular phase-shifter positions due to the inevitable resonance condition and is therefore unacceptable.
In order to design a phase-tunable feed network that allows antenna-beam steering and beam-width variation with sufficient performance at adequate costs, a principal design has to be found without the drawbacks of the prior art.